It is generally known that telephone service is sometimes abused by persons making fraudulent calls, that is, calls that cannot be billed properly. One method of placing a fraudulent call involves the use of the so-called "blue box" device. The blue box device permits a calling party to simulate in-band supervisory signals and multifrequency tones which are normally generated by the switching equipment to direct calls through the toll network. The blue box user generally places an inexpensive toll call to a nearby destination or a call to a toll free number. At the start of ringing when the message billing equipment is waiting for an answer signal from the destination office, the caller transmits a disconnect signal to the destination toll office. In response to the signal, the destination office abandons the call in progress. The blue box user now transmits a seizure signal which is interpreted at the destination toll office as a new request for service. The destination office attaches a sender to the connection and returns a "proceed-to-send" signal to the calling party. The blue box caller can now key in a new number and direct this second call anywhere in the toll network. The message billing equipment at the originating office is unaware of the second call and charges the calling customer on the basis of the inexpensive or free call to the first number.
Attempts have been made to detect calls from a blue box user. In one known arrangement such as disclosed in U.S. Pat. No. 4,002,848 to R. F. Stein of Jan. 11, 1977, tone detectors are connected at a point in the speech path where in-band signaling and multifrequency switching and control tones normally are not present. If tones are detected at this point, it is presumed that the tones are being generated by a blue box at the calling station and a record is made of the call details for further investigation. Another known method, such as disclosed in U.S. Pat. No. 4,001,513 to D. C. Naylor of Jan. 4, 1977, recognizes the second proceed-to-send signal which occurs at the time when answer supervision is normally expected from the called customer. In this arrangement, a multifrequency receiver is coupled to the connection to record the tones transmitted by the calling customer's blue box.
While both of the above arrangements are suitable for their intended purpose, they require a multiplicity of tone receivers and apparatus to couple the receivers to the connection. Moreover, these prior art arrangements are not suitable for detecting the so-called "black box" fraud device which does not use multifrequency tones. This device permits free calls to be made to the black box user because the device returns a short off-hook signal to the calling office without establishing a charging condition on the trunk.
Thus, a need exists in the prior art for detecting irregular toll calls that may not be accompanied by tones fraudulently transmitted by the calling customer.